Acoustic Neuromas

It is not uncommon for people to experience some loss of hearing with age.  The condition, known as “presbycusis” affects about thirty to thirty-five percent of all Americans over the age of 60 and about half of those over 85.  Caused mostly by wear and tear on the components of the inner ear, in most people, the gradual loss of hearing arises mostly from exposure to loud noises although sometimes simple factors such as blockage of the ear passages by earwax may be the problem.

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Most hearing loss is permanent, but hearing devices can help to improve the condition.  One rare form of hearing loss is known as acoustic neuromas, also called vestibular schwannomas because it affects the Schwann cells of the inferior vestibular nerve (Komatsuzaki and Tsunoda, 2001) which innervates the posterior semicircular canal and saccule of the ears.  Schwann cells cover the axons of nerve cells or “neurons”.  The condition is not common.  It basically occurs in adults.  There are only 39 total cases reported of the disease in children in the literature (Pothula et al, 2001) and about 2,000 to 3,000 new cases in the U.S. each year in the general population. (Evans et al, 2005)  Even in adults, the National Institute of Health (NIH, 1991) reports that only 1 person in 1,000 has acoustic neuroma.

 Acoustic neuromas are rare tumors associated with the ears.  These tumors generally cause hearing loss on one side and may be associated with dizziness as well as other symptoms of brain involvement.  There may also be facial twitching and large tumors may lead to a disturbance of the sensory facial apparatus and perhaps 40 percent may lead to headaches.

About 10% of all acoustic neuromas present as sudden deafness. (Sataloff and Sataloff, 2005).  Sataloff and Sataloff categorize eight general causes of hearing loss all of which involve some degree of recruitment and hearing distortion in the early stages.  Their eight categories are Meniere’s disease, prolonged exposure to intense noise, acoustic trauma, direct head trauma, ototoxic drugs, viral infections, labyrinthosis following ear surgery and congenital cochlear disease.  Of these categories, only “acoustic trauma” is of interest this review.

“Acoustic Neuroma” refers to a benign tumor of the Schwann cell that surrounds the vestibular portion of the eighth cranial nerve, the vestibulocochlear nerve, or just the vestibular nerve.  Kim and Jenkins state, “Cerebellopontine angle (CPA) lesions are the predominant skull base neoplasms that affect the posterior fossa. ‘Acoustic neuromas’ account for most (vestibular schwannomas), but other varieties of lesions must be considered in the differential diagnosis, including meningiomas, primar cholesteatomas, other cranial nerve schwannomas, arachnoid cysts, hemangiomas, metastases, and paragangliomas.” (p. 461)

There are two forms of the disease, a sporadic form which includes about 95% of all the cases and a rare inherited form associated with a syndrome referred to as neurofibromatosis type II (NF 2).  Inherited acoustic neuromas are rare.  The attending physician should suspect acoustic neuroma caused by type-II neurofibromatosis in young patients where there is a family history of neural tumors.  Acoustic neuromas are non-malignant tumors that affect the 8th cranial nerve, the vestibular nerve.

However, they are thought to be most common in the Schwann cell of the inferior vestibular nerve. (Komatsuzaki and Tsunoda, 2001)  This issue has not been settled.  Acoustic neuromas also arise in the labyrinth of the ear (Neff et al, 2003), the fluid filled semicircular canals or tubules in the inner ear.  The labyrinths of the ears are commonly associated with balance problems or vertigo and individuals presenting with vertigo often undergo tests for acoustic neuroma, but acoustic neuroma rarely causes vertigo. (Selesnick et. al., 1993)  Vertigo only occurs in about 20 percent of acoustic neuroma patients. (Komatsuzaki and Tsunoda, 2001)  When present, vertigo is most common in situations that involve small tumors, but perhaps 70 percent of those patients with large acoustic neuromas may be unsteady.  Unsteadiness and poor coordination of the arms suggestive of tumors in the cerebellum are not common.

Acoustic neuromas comprise about 6 percent of all intracranial tumors, but they also comprise about 30% of all brainstem tumors and 85% of the tumors in the region of the cerebello-pontine angle (CPA) (Evans et al, 2005), the region where the cerebellum meets the pons.  Acoustic neuromas are the most common of all lesions of the cerebello-pontine angle accounting for 70 to 90 percent of all CPA lesions.  Some 10% of acoustic neuromas are in the meninges that cover the brain and are therefore meningiomas.

More than 95 percent of all patients with acoustic neuromas have slowly progressive, usually, unilaterally hearing loss.  Hearing loss caused by acoustic neuromas may be sudden in as many as one quarter of all cases, but that would only account for perhaps 5 percent or less in the general population of all patients with a sudden hearing loss from all causes. (Daniels et al, 2000)  Unilateral tinnitus, a ringing in the ears, is also very common in acoustic neruoma.

About two-thirds of acoustic neuroma patients there is a loss of the ability to hear high-frequency sounds.  Occasionally the disease is associated with congenital hearing loss, but most of the remaining individuals with the neuroma have a problem hearing low frequency sounds.  Even though hearing loss is generally permanent, there have been cases of complete hearing recovery in hearing loss caused by acoustic neuromas (Nageris and Popovtzer, 2003) and there appears to be cases where acoustic neuromas may not affect hearing at all. (Morrison and Sterkers, 1996)

Treatment:

Acoustic neuromas are generally detected by testing the hearing using conventional audiometry and by using MRI scans of the head with gadolinium contrast.  When conventional audiometry indicates the need for further testing, tests of the auditory brainstem response (ABR or BAER for brainstem auditory evoked response) may provide further information.  This decision is one of both cost and need.  While ABR testing is less sensitive than resorting to gadolinium enhanced MRI (magnetic resonance imaging), it is also much less expensive.  Although acoustic neuromas are generally unilateral in nature, symmetrical hearing loss does not exclude the existence of a large acoustic tumor on one side.  MRI can give a definitive resolution to this issue, but the cost is a factor in determining whether to use MRI.

Like the heart with its EKGs and the brain with its EEGs, hearing can be tested with ENGs (electronystagmography).  The ENG is abnormal in about 50 percent of all patients with tumors that generate a unilateral loss of hearing.  However, ENG testing alone is not sufficient to make a diagnosis about acoustic neuromas.  Although there are several forms of treatment for acoustic neuromas, it is unlikely than any form of treatment will preserve hearing.  Perhaps one half of all acoustic neuromas are surgically treated and around one quarter are treated with radiation.  Some may not be treated at all, but only watched.

Since CAT (or CT) scans and MRI scans have come into use, it is becoming more common to discover acoustic neuromas accidentally such as when investigating to determine why a patient is suffering from migraine headaches.  In fact, acoustic tumors may be accidentally found when a patient is being observed from any one of a number of other situations.

The management and care of patients with acoustic neuroma changed dramatically during the 20th century thanks to technical advances and microsurgical techniques.  Today, doctors can reduce the mortality and morbidity associated with the disease, preserve facial nerve function as there is a connection between the facial nerve and acoustic neuroma, and sometimes save hearing.

The primary method to treat acoustic neuroma is through surgery, but radiation therapy is used in most cases where surgery is not a good option.  Treatment of acoustic tumors with surgery or radiation will lead to vestibular imbalance and/or deafness.  In situations where acoustic tumors are discovered, it may be best to obtain evidence that the tumor is growing before taking the route of surgery or radiation treatment, and this may be a good situation to obtain a second opinion.

Patient Outcomes

Patients with acoustic neuroma may survive without hearing loss, and in fact, it is possible to save hearing loss due to acoustic neuromas.  The facial nerve is part of the nerve complex associated with the internal auditory canal and may be damaged during surgery for acoustic neuroma.  There is some facial nerve paralysis associated with treatment for acoustic neuroma and at one time such damage was routine, but unlike the past, today most patients with the disease can avoid any facial paralysis or damage to the facial nerve.  Still, there are other complications associated with acoustic neuromas.

Today, microsurgical techniques have low morbidity and mortality rates.  The most common side effect that arises from treating and managing acoustic neuromas is a complication by vertigo and a loss of equilibrium but patients rapidly compensate for the vestibular loss.  Immediately after surgery, there may be bleeding and hematoma formation in the CPA.  Cerebellar damage and edema can be a fatal complication from surgical intervention.  The most common complication from surgical treatment of acoustic neuroma is the leaking of the cerebrospinal fluid (CSF).  Meningitis is also possible due to the leaking CSF.  If the facial nerve is transected during surgery, it can be managed.  Patients who have had the surgery are at risk for peptic ulcers, pulmonary embolism, a syndrome of inappropriate secretion of antidiuretic hormone (SIADH) and there are other risks associated with the surgery.

Summary:

Acoustic neuromas are rare tumors associated with a unilateral hearing loss.  The disease, though rare in adults, is much less common in children with only 39 cases reported in the literature.  There are two forms of the disease, one that is the common form and a second, much less common form that might be inherited.  Acoustic neuromas are detected by means of conventional audiometric tests and by use of CAT scans and MRI scans.  The most common means of treating the disease is to surgically remove the tumor but radiation treatment is also used.  Sometimes the tumor is left alone.

Patients with acoustic neuroma may survive without hearing loss and in fact, sometimes the situation is only discovered accidentally when examining the patient for something else.  However, the surgical removal of the tumor may result in damage to the facial nerve which can cause facial paralysis and it may also result in leaking CSF that can lead to meningitis.  Hearing loss when it occurs is generally permanent.